1. Field of the Invention
The present invention relates to integrated electronic circuits, and, in particular, to current mirrors.
2. Description of the Related Art
A current mirror is a type of current amplifier that provides a high impedance output current proportional to an input signal current. The output current may be used, for example, to drive a load for high gain.
FIG. 1 shows a representation of a simple current mirror 100 consisting of a single input transistor 102 and a single output transistor 104, with the gate electrodes G of the pair being tied together and to an input voltage node V which is common to both. The sources of the transistors are connected to a reference voltage node which is common to both (e.g., ground). The drain and the gate of the input transistor are connected to an input signal current source I.sub.IN that provides a reference current. Because the input and output transistors have their gates tied together and their sources tied together, a corresponding output current I.sub.OUT arises in the conduction path of the output transistor. In some current mirrors, the input and output transistors are identical (e.g., have the same width-to-length ratio) and there is a substantially unity gain in the current. Such current mirrors are commonly used to provide active loads in high-gain amplifier stages. Unfortunately, the output impedance of such current mirrors is too low for many applications.
U.S. Pat. No. 4,477,782 (Swanson), the teachings of which are incorporated herein by reference, discloses current mirrors in which two input signal currents are used to generate the output current. In certain applications, it may be inefficient to use a current mirror that uses two input signal currents.
U.S. Pat. No. 4,550,284 (Sooch), the teachings of which are incorporated herein by reference, discloses current mirrors in which, during operation, some of the transistors are in the saturation region, while other transistors are in the triode region. In order to ensure proper operation, the transistor model parameters in Sooch need to be consistent from transistor to transistor.
It is an aspect of the present invention to provide a current mirror that does not suffer from the limitations of the prior art of having either two input signal currents or transistors that operate in different regions.
Further aspects and advantages of this invention will become apparent from the detailed description which follows.